Method of producing a boron suboxide material

ABSTRACT

A method of producing a boron suboxide composite material having improved fracture toughness.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a division of U.S. Pat. No. 7,955,579 issued Jun. 7,2011 entitled “Boron Suboxide Composite Material” which is a 371application of PCT/IB2006/002456 filed Sep. 6, 2006, published on Mar.15, 2007 under publication number WO 2007/029102 A and claims prioritybenefits of South African Patent Application Number 2005/07180 filedSep. 7, 2005, the disclosures of which are incorporated herein byreference.

BACKGROUND OF THE INVENTION

The invention relates to a boron suboxide composite and to a method forits preparation.

The first laboratory synthesis of diamond triggered extensive efforts todesign and develop materials with a combination of propertiesapproaching or even improving upon those of diamond. The best known ofthese superhard materials is cubic boron nitride (cBN). It is also knownthat boron rich compounds provide good candidates for this type ofapplication. They give rise to a large family of refractory materialswith unique crystal structures and a range of interesting physical andchemical properties related to their short interatomic bond lengths andtheir strongly covalent character. Boron rich phases with a structurebased on that of α-rhombohedral boron include boron carbide and boronsuboxide (nominally B₆O), which combine high hardness with low densityand chemical inertness, making them useful as abrasives and for otherhigh-wear applications [1 ].

The boron suboxide (B₆O) structure, space group R 3m, consists of eightB₁₂ icosahedral units situated at the vertices of a rhombohedral unitcell. The structure can be viewed as a distorted cubic close packing(ccp) of B₁₂ icosahedra. Two O atoms are located in the intersticesalong the [111] rhombohedral direction.

The synthesis of boron suboxide (B₆O) and a description of itsproperties have been extensively reported in the literature, even thoughpure material with a high degree of crystallinity is difficult tosynthesize. Boron suboxide materials formed at or near ambient pressureare generally oxygen deficient (B₆O_(x), x<0.9). They also have poorcrystallinity and very small grain size. High pressure applied duringthe synthesis of B₆O can significantly increase the crystallinity,oxygen stoichiometry, and crystal size of the products [1]. Althoughboron suboxide is reported as the nominal composition B₆O, it is widelyaccepted to be non-stoichiometric. For brevity, the nominal formula B₆Ois used in this specification.

In U.S. Pat. No. 3,660,031 a method of preparing boron suboxide isdisclosed. According to this disclosure, the boron suboxide is formed byreducing zinc oxide with elemental boron at a temperature in the rangeof 1200° C. to 1500° C. It is reported as having the formula B₇O, and isalso characterized as having an average hardness value of 38.20 GPaunder a load of 100 g, and a density of 2.60 g/cm³. The fracturetoughness of this material is not reported.

U.S. Pat. No. 3,816,586 also discloses a method of fabricating boronsuboxide. According to this disclosure, boron suboxide is formed by hotpressing the mixture of elemental boron and boron oxide at suitabletemperatures and pressures. Upon analysis, the boron suboxide product issaid to have given 80.1 wt. % boron and 19.9 wt. % oxygen whichcorresponds to the stoichiometry of B₆O. It is also reported as having adensity of 2.60 g/cm³ and a Knoop hardness under a 100 g load (KNH₁₀₀)of 30 GPa. The fracture toughness of this material is not reported.

A great deal of research has shown that while boron suboxide materialhas a very high hardness its fracture toughness is very low, i.e. thematerial is brittle. From the literature, Itoh et. al. [2], B₆O compactshave been manufactured at high temperatures (1400° C.-1800° C.) and highpressures (3-6 GPa). This B₆O powder is reported to have beensynthesized from elemental boron and boric oxide. Upon analysis, the B₆Ocompacts are reported as having an average hardness of 31-33 GPa and avery low fracture toughness. Itoh et. al. [3, 4] and Sasai et. al. [5],have also tried to improve the mechanical properties of B₆O, especiallyfracture toughness, using other hard materials like cBN [3], boroncarbide [4], and diamond [5], respectively. The hardness for these B₆Ocomposites is respectable but the fracture toughness is reported to bestill low, B₆O-diamond composites having a fracture toughness of about 1MPa·m^(0.5), B₆O-cBN composites having a fracture toughness of about 1.8MPa·m^(0.5) and B₆O—B₄C composites having a fracture toughness of about1 MPa·m^(0.5).

It is an object of the present invention to provide a method ofproducing B₆O composites with a respectable hardness as well as a betterfracture toughness, compared to the previously reported B₆O composites.

SUMMARY OF THE INVENTION

According to one aspect of the invention, there is provided a boronsuboxide composite material comprising particulate or granular boronsuboxide distributed in a binder phase comprising M_(x)B_(y)O_(z),wherein

-   -   M is a metal;    -   X is from 4 to 18;    -   Y is from 2 to 4;    -   Z is from 9 to 33.

The metal is preferably selected from the group comprising aluminium,zirconium, titanium, magnesium and gallium, in particular aluminium.

The boron suboxide preferably comprises greater than 70% by weight ofthe composite material, in particular from about 85 to about 97% byweight.

The binder phase preferably comprises less than about 30% of thecomposite material, in particular from about 3 to about 15% by weight.

The composite material of the invention preferably has a fracturetoughness of greater than about 2.5 MPa·m^(0.5).

According to another aspect of the invention, a method of producing aboron suboxide composite material includes the steps of providing asource of boron suboxide particles, preferably a powder, coating theboron suboxide particles with a metal or metal compound, preferably bychemical vapour deposition, and sintering the metal coated boronsuboxide particles at a temperature and pressure suitable to produce acomposite material.

The metal is preferably selected from the group comprising aluminium,zirconium, titanium, magnesium, and gallium, in particular aluminium, orcompounds thereof. The sintering of the metal-coated boron suboxideparticles is preferably carried out using a hot press, preferably at atemperature of greater than about 1600° C., in particular at atemperature of about 1900° C., and preferably at a pressure of less thanabout 300 MPa, in particular at a pressure of about 50 MPa.

An activator may be used during coating of the boron suboxide particles.For example, in the case of aluminium, ammonium chloride may be used asan activator during the coating of the boron suboxide particles.

DESCRIPTION OF THE EMBODIMENTS

The boron suboxide composite material of the present invention is madeby hot pressing a metal-coated B₆O powder at high temperatures and lowpressures.

The starting B₆O powder is coated with a metal, in this case aluminium,using a chemical vapor deposition (CVD) process, at moderately hightemperatures. For example, aluminium (Al) powder is admixed withammonium chloride (NH₄Cl) using a turbular mixer with alumina balls forseveral hours, typically 1.5 hours. Ammonium chloride is used as anactivator during coating. As a large amount of aluminium may reduce B₆Oduring coating, a very small amount of aluminium is admixed withammonium chloride. After this admixing, the milled B₆O powder is admixedwith the mixture (Al/NH₄Cl), using alumina balls for several hours,typically 1.5 hours. The new mixture (B₆O/Al/NH₄Cl) is poured into analumina boat, and alumina pebbles placed on top of the mixture. Thealumina pebbles act as inert fillers and capture some of the gases beingreleased in the furnace, which prevents the clogging of the exhaust pipeof the tube furnace. The alumina boats are then placed in the tubefurnace, and heated up to about 1400° C., with a low heating rate.

This CVD process provides particles coated with Al—B—O compounds, in ahomogeneous distribution. Although aluminium is described forconvenience, it is to be understood that the process can also be carriedout by coating the B₆O starting material using other metal compounds,such as zirconium, titanium, magnesium and gallium, for example. Theresulting coated B₆O powder is sintered at high temperatures (about1900° C.) and low pressures (about 50 MPa), using a hot press. Firstly,the coated powder is poured into a boron nitride cell, which is thenplaced inside a graphite die. The sintering is typically carried outunder argon or other inert atmosphere.

The resulting material can then be characterised, typically using X-raydiffraction, scanning electron microscopy, optical microscopy anddensity determination using Archimedes principle. The boron suboxidecomposite made in this manner is found to have good mechanicalproperties and a fracture toughness of greater than 2.5 MPa·m^(0.5), andup to about 5 MPa·m^(0.5).

Without wishing to be bound by theory, it is believed that the improvedfracture toughness of the boron suboxide composite materials of theinvention is due to the effect of the metal at the grain boundaries ofthe boron suboxide particles during sintering.

It has been found that the fracture toughness of pure sintered B₆O isvery low. It is well known that borides-based particles typically have athin B₂O₃ coat on them. When sintering such particles the B₂O₃ phase,which is quite weak, remains at the grain boundaries. The presence ofsuch a weak phase at the grain boundaries weakens the material and makesit very easy for a crack to propagate through it. Coating the particleswith a M-B—O based phase results in the weak B₂O₃ being replaced by amuch stronger M_(x)B_(y)O_(z) phase. As a result crack growth byintergranular fracture is now much more difficult.

A secondary reason for the increase in fracture toughness of thismaterial is believed to lie in the fact that the B₆O phase and theM_(x)B_(y)O_(z) phases possess different thermal expansion coefficientsand elastic constants. As a result of this difference in properties,when the material is cooled down after sintering, bimetallic stressesare set up between the two different phases. The presence of suchstresses, which can be very high, can cause deflections of a propagatingcrack, thus making said propagation more energetically expensive, thusincreasing the material's fracture toughness.

The invention will now be described with reference to the followingnon-limiting examples.

Example 1

The B₆O powder starting material was milled using a planetary ball millwith alumina balls for about four hours. The alumina balls were used asthe B₆O powder was to be coated with aluminium so alumina ascontamination was regarded as being acceptable. The amount of alumina inthe milled B₆O powder was less than 1%, and was therefore ignored. Theintroduction of the milling stage helped in breaking down theagglomerates which were present in the powder.

Aluminium powder (5 microns) was admixed with ammonium chloride (NH₄Cl)for one and a half hours using a turbular mixer with alumina balls toprovide the coating material, the ammonium chloride being used as anactivator during coating. As large amounts of aluminium could reduce theB₆O during coating, for initial experiments only 20 vol. % of Al and 80vol. % of NH₄Cl were used.

After the first admixing, the milled B₆O powder was admixed with themixture (Al/NH₄Cl), using alumina balls for one and a half hours,according to the mass ratio of 4:0.3 (B₆O:Al/NH₄Cl). The mixture(B₆O/Al/NH₄Cl) was poured into an alumina boat, and alumina pebbles wereplaced on top of the mixture to act as inert fillers and capture some ofthe gases being released in the furnace, which prevents the clogging ofthe exhaust pipe of the tube furnace. The alumina boat was placed in thetube furnace, and heated up to 1400° C., at a rate of 10° C./min. As theformation of AlCl₃ and release of gases occurred at around 350° C.,there was a dwelling point of one hour at this temperature. During thatperiod of time, the following reaction took place:Al(s)+3NH₄Cl(s)->AlCl₃(s)+3NH₃+3/2H₂(g)

A second dwelling time of six hours was maintained at 1400° C. in orderto allow for a complete coating process, and then followed by cooling,at a rate of 10° C./min. The coated B₆O powder contained alumina andsome traces of aluminium boride (AlB₁₂). It was then placed in a boronnitride cell (inside a graphite die) and sintered using a hot press at atemperature of 1900° C. and a pressure of 50 MPa, under an argonatmosphere, for about 20 minutes.

The boron suboxide composite made in this manner was found to have ahardness of 29 GPa at a load of 5 kg, which compares favourably withthat of the prior art. Most importantly, however, the composite materialof the invention was found to have a fracture toughness value of about 3MPa·m^(0.5), which is believed to be greater than any previouslyreported value for a boron suboxide composite.

Example 2

The conditions of Example 1 were repeated except that this time the massof Al/NH₄Cl mixture was increased. The mass ratio of the mixed powderwas 4:0.5 (B₆O:Al/NH₄Cl). The coating and hot pressing conditions usedin Example 1 were used in preparing this sample. The resultant samplewas polished and then tested for hardness and fracture toughness with aVickers indenter, and was found to have a hardness (5 kg load) of about25 to 28 GPa and a fracture toughness of about 3.5 MPa·m^(0.5).

Example 3

The conditions of Example 1 were repeated except that the mass ofAl/NH₄Cl mixture was again increased. The mass ratio of the mixed powderwas 4:0.75 (B₆O:Al/NH₄Cl). The coating and hot pressing conditions usedin Example 1 were used in preparing this sample. The resultant samplewas polished and then tested for hardness and fracture toughness with aVickers indenter, and was found to have a hardness (5 kg load) of about24 to 27 GPa and a fracture toughness of 3.5 MPa·m^(0.5).

Example 4

The conditions of Example 1 were repeated and the mass of Al/NH₄Clmixture was once again increased. The mass ratio of the mixed powder was4:1 (B₆O:Al/NH₄Cl). The coating and hot pressing conditions used inExample 1 were used in preparing this sample. The resultant sample waspolished and then tested for hardness and fracture toughness with aVickers indenter, and was found to have a hardness (5 kg load) of about24.5 GPa and a fracture toughness of 4.75 MPa·m^(0.5).

In order to confirm the reproducibility of the results obtained in theabove Examples, a new batch of synthesized B₆O was coated using the same(coating and hot pressing) conditions as in Example 1. Table 1 belowsets out all the results, including the repeated samples. There is aslight difference in properties of some of the hot pressed B₆Ocomposites (with same Al content), but that was attributed to thedifference in densities, which is also partly connected with surfaceporosity or decomposition.

The hot pressed B₆O composites of the invention had higher fracturetoughness figures compared to a hot pressed “pure” B₆O material,Comparative Example in Table 1, as a result of strengthening caused bythe formation of aluminium borates after sintering.

TABLE 1 B₆O: Phases Phases Al/NH₄Cl Density K_(IC) (after (after (massin g) (g/cm³) Hv₅ (GPa) (MPa · m^(1/2)) coating) sintering) Comparative0 wt % Al 2.51 30.1 ± 1.2  Brittle B₆O B₆O Example (pure B₆O) (1 kgload) Ex 1 4:0.3 2.49 29.3 ± 0.30 2.98 ± 0.16 B₆O B₆O Repeated 2.52 29.3± 0.47 2.71 ± 0.43 Al₂O₃ Al₄B₂O₉ Sample Al₄B₂O₉* (2.2 wt % Al) Ex 24:0.5 2.42 25.3 ± 0.35 3.88 ± 0.23 B₆O B₆O Repeated 2.45 28.2 ± 1.553.25 ± 0.96 Al₂O₃ Al₄B₂O₉ Sample Al₄B₂O₉* (3.76 wt % Al) Ex 3 4:0.752.39 24.3 ± 0.24 4.22 ± 0.30 B₆O B₆O Repeated 2.51 27.8 ± 1.11 3.45 ±0.12 Al₂O₃ Al₄B₂O₉ Sample Al₄B₂O₉* (5.6 wt % Al) Ex 4 4:1 2.37 24.5 ±0.78 4.75 ± 0.25 B₆O B₆O Al₂O₃ Al₄B₂O₉ Al₄B₂O₉* *Traces of Al₄B₂O₉

REFERENCES

-   1. H. Hubert, L. Garvie, B. Devouard, P. Buseck, W. Petuskey, P.    McMillan, Chemistry of materials, 10, pg 1530-1537, 1998.-   2. H. Itoh, I. Maekawa and H. Iwahara, Journal of Material Science    Society, Japan, 47, No. 10, pg. 1000-1005, 1998.-   3. H. Itoh, R. Yamamoto, and H. Iwahara, Journal of American Ceramic    Society, 83, pg. 501-506, 2000.-   4. H. Itoh, I. Maekawa and H. Iwahara, Journal of Material Science,    35, pg. 693-698, 2000.-   5. R. Sasai, H. Fukatsu, T. Kojima, and H. Itoh, Journal of Material    Science, 36, pg. 5339-5343, 2001.

1. A method of producing a boron suboxide composite material includesthe steps of providing a source of boron suboxide particles, coating theboron suboxide particles with a metal or metal compound, and sinteringthe metal coated boron suboxide particles at a temperature and pressuresuitable to produce a composite material having a fracture toughness ofgreater than 2.5 MPa·m^(0.5).
 2. A method according to claim 1 whereinthe boron suboxide particles are provided in powder form.
 3. A methodaccording to claim 1, wherein the boron suboxide particles are coated bychemical vapour deposition.
 4. A method according to claim 1, whereinthe metal is selected from the group comprising aluminium, zirconium,titanium, magnesium, and gallium, or compounds thereof.
 5. A methodaccording to claim 1, wherein the metal is aluminium, or a compoundthereof.
 6. A method according to claim 1, wherein the sintering of themetal-coated boron suboxide particles is carried out using a hot pressat a temperature of greater than 1600° C. and at a pressure of less than300 MPa.
 7. A method according to claim 6, wherein the sintering iscarried out at a temperature of 1900° C. and a pressure of 50 MPa.